The technical field is rewritable optical storage media, and in particular, rewritable digital video discs.
A timing signal recovery circuit is needed to record and read data on an optical storage device such as a digital video disc. The optical storage device may include a pilot tone, or reference signal. An example of such a reference signal is a wobble in the path of a groove on a disc for optical recording.
A raw wobble signal extracted from tracking or other optical detectors in the optical data storage device contains, in addition to the desired timing reference signal with its modulation, noise and interference from the main data recording and play back function and noise and interference from adjacent tracks. If the optical data storage device is operating in the desirable mode of constant angular velocity, then the center frequency range of the timing reference signal to be recovered is wide, greater than one octave, and typically 2.5 to 1. In order to isolate the timing reference signal with minimal jitter, and to demodulate the address or other information carried by this signal, filtering and synchronous detection with phase locked carrier regeneration can approach optimal performance. Satisfying these criteria places costly constraints on the phase linearity of at least one of the required filters.
A circuit for recovering the timing reference signal includes a superheterodyne phase locked loop. The recovery circuit avoids the need for a phase linear filter with a large percentage bandwidth and allows placement of the filter at a point in the recovery circuit where the center frequency to be passed is substantially fixed. The recovery circuit thus constructed further allows the bandwidth of the filter to be narrower and tailored to match the modulation characteristics of the timing recovery signal. This further results in a better signal-to-noise ratio into a limiter and phase detector, improving the performance of the phase locked loop.
In an embodiment, the recovery circuit includes a high frequency reference oscillator that supplies a reference signal for use with the timing circuit. The reference signal is applied to a divide by M stage, where M relates to the number of data bits per wobble cycle. In this embodiment, the value of M is fixed. The thus-divided reference signal is further applied to a divide by 4 stage to produce an in-phase and a quadrature-phase signal for mixing in a superheterodyne mixer circuit. The output of a second mixer provides a timing signal 1/T.
In an alternative embodiment, a timing recovery circuit that provides more flexible tracking and control, and that uses a lower frequency local (reference) oscillator includes two loops, Loop A and Loop B. Loop A performs a superheterodyne tracking of a wobble signal, and outputs a regenerated wobble signal to Loop B. Loop B is a synthesizer loop that generates a main data clock frequency 1/T and a signal 4/T. Loop B also generates a recovered wobble frequency signal 1/MT, which may be used as a wobble timing output. The timing recovery circuit eliminates divider stages that may generate ambiguities and thus require synchronization.
A further advantage of the timing recovery circuit is that Loops A and B are in cascade but do not interact, allowing their characteristics to be independently adjusted. The overall transfer function of the timing recovery circuit will then be the cascade combination of Loops A and B. For example, Loop A may be set to have a narrow bandwidth to accomplish most of the needed filtering. Loop B can then be given a wide bandwidth for ease of acquiring lock. Alternatively, Loops A and B might be given similar bandwidths to use the cascade for optimal filtering. Because of the independent arrangement of the Loops A and B, the timing recovery circuit can operate with optical storage devices having different values of M.